1. Field of the Invention
This invention relates to an image processing device and a compression processing method and more particularly to an image processing device for performing image processing for page description data received from an external device and preparing output image data and a compression processing method of compressing image data prepared by performing image processing for page description data representing an image.
2. Description of the Related Art
The term "band buffer" in the specification is used to mean a memory for storing image data, corresponding to each of divisions into which a 1-page image area is divided.
Hitherto, an image processing device has been widely used which receives page description data represented in a page description language from an external information processing device, etc., and performs image processing for the received page description data for preparing image data to be printed, then transmits the prepared image data to an output unit such as a laster beam printer or an ink jet printer, which then prints an image on paper. To store a large amount of image data to be processed in a finite internal memory, the image processing device uses a technique of holding image data at an intermediate stage (intermediate data) in a compression state.
In processing wherein all 1-page page description data is converted into intermediate data and then the intermediate data is rasterized in synchronization with the output processing performance of an output unit and is output from the output unit, an art of determining whether or not the intermediate data is to be compressed based on the rasterization processing time and the empty capacity of a memory and compressing the intermediate data whenever necessary in response to the determination result is proposed, for example, in Japanese Patent Laid-Open No. Hei 7-195753.
By the way, in recent years, high-speed color printers have been developed and are becoming pervasive rapidly meeting demands for color image printing and speeding up the printing.
However, to apply the art described in Japanese Patent Laid-Open No. Hei 7-195753 to such a high-speed color printer and rasterize and output intermediate data in synchronization with the output processing performance of the color printer, a bus width about three times that of the output processing performance of the color printer becomes necessary for an ESS (electric device) in the image processing device; the image processing device needs to be provided with special hardware.
Then, before transmitting image data to an output unit, the image processing device can adopt an art of compressing rasterized image data and storing the compressed data in a compressed page buffer, then decomposing the stored data in synchronization with the output processing performance of the output unit and transmitting the data to the output unit at the print-out time.
An art of dividing a 1-page image area into subareas and preparing image data from page description data and compressing the data for each subarea is proposed, for example, in Japanese Patent Laid-Open No. Hei 5-31974.
By the way, data represented by page description data is roughly classified into the three types: Characters (text), graphics, and images, such as images read through a scanner. A large-capacity memory is required to perform image processing for page description data representing an image. Particularly, to rotate an image by an angle other than 90 or 180 degrees (arbitrary angle rotation processing) in a print image area in image processing, for example, as shown in FIG. 3, image area Q fitted within one band buffer width L extends across three band buffers (bands 0, 1, and 2 in FIG. 3), a large-capacity memory area being required.
However, the art described in Japanese Patent Laid-Open No. Hei 5-31974 does not consider large-capacity memory consumption in image processing for the page description data representing an image.
To actually perform arbitrary angle rotation processing as shown in FIG. 3, for example, a sequence of steps of performing arbitrary angle rotation processing for the page description data representing an image little by little (in small scan line units), compressing image data expanded in band buffers, temporarily storing the compressed data in the memory, and before completion of the arbitrary angle rotation processing for all data, decomposing the already compressed image data little by little and transmitting the decomposed data to a printer must be executed to use the finite memory built in the image processing device. Thus, it becomes necessary to repeat compressing and decompressing because of memory shortage, leading to worsening of processing efficiency and degradation of image quality.